1. Field of the Invention
The present invention is related to the field of armored cables used in electrical logging of oil and gas wells. More specifically, the present invention is related to a design for a combination electrical and fiber-optic cable used to lower logging instruments into wellbores drilled into the earth.
2. Description of the Related Art
Well logging is known in the an for providing measurements of properties of earth formations penetrated by wellbores. Well logging includes inserting measuring instruments into the wellbore, the instruments being connected to one end of an armored electrical cable. The armored electrical cables known in the art typically comprise at least one insulated electrical conductor which is used both to supply electrical power to the instruments and to transmit signals generated by the instruments to other equipment located at the earth's surface for decoding and interpreting the signals. The cables known in the art further comprise steel armor wires helically wound around the electrical conductor to provide tensile strength and abrasion resistance to the cable.
The signals generated by the instruments for transmission to the earth's surface are typically electrical signals. The signals can be in the form of analog voltages or digital pulses. A drawback to using electrical signals in well logging is that the mechanical requirements imposed on the logging cable, for example, relatively high bending flexibility and low weight per unit length, require that the cable and the electrical conductor be formed from wires generally having small diameter. A typical well logging cable, for example, comprises an electrical conductor consisting of seven strands of 0.0128 inch diameter copper wire covered by a 0.096 inch external diameter plastic insulator. The electrical conductor is typically characterized by a resistance of about 9 ohms per 1,000 feet of conductor and has a capacitance of several picofarads per foot of conductor.
Other cables known in the art can include a plurality of conductors arranged in a central bundle, each conductor having about the same construction and electrical characteristics as the conductor used in the single conductor cable.
Because of the electrical properties of the conductors in typical well logging cables, the well logging cables known in the art typically cannot effectively transmit electrical signals at frequencies above 100 kilohertz (kHz). Newer types of well logging instruments can generate data at rates which make using electrical signal transmission difficult and expensive.
It is known in the art to provide optical fibers in well logging cables to enable use of optical telemetry, which is capable of much higher frequencies and data transmission rates than is electrical signal transmission. For example, U.S. Pat. No. 4,696,542 issued to Thompson, describes a well logging cable having optical fibers disposed substantially centrally within helically-wound, copper-clad steel conductors, the conductors themselves covered by two layers of contra-helically wound steel armor wires. A drawback to the well logging cable described in the Thompson '542 patent is that the optical fibers are encased in a plastic tube. Well logging cables can be exposed to hydrostatic pressures and to temperatures in the wellbore which are high enough to preclude the use of the plastic tube as disclosed in the Thompson '542 patent.
Other fiber optic cables known in the art include enclosing the optical fibers in a steel tube. For example, "Electro-Optical Mechanical Umbilicals", Vector Cable, Sugar Land, Tex. (publication date unknown) discloses several so-called "towing and umbilical" cables which include steel tubes enclosing the optical fibers. A drawback to the combination electrical/optical fiber cables described in the vector Cable reference is that the cable designs disclosed therein have very large diameter electrical conductors which are intended to be used only for electrical power transmission; the optical fibers perform substantially all the signal communication functions of the cable. For reasons known to those skilled in the art, use of the large diameter power conductors as disclosed in the Vector Cable reference results in a cable having such a large external diameter that use of certain fluid pressure control equipment is precluded.
The cables disclosed in the Vector Cable reference also have substantially different electrical signal transmission characteristics than do well logging cables known in the an because of the large size of the power conductors. It is also desirable to provide a combination electrical/fiber optic cable having electrical conductors capable of maintaining the electrical signal transmission capabilities of the electrical logging cables known in the an so that existing well logging instruments using electrical telemetry need not be redesigned.
A combination fiber-optic/electrical well logging cable having the optical fiber enclosed in a steel tube is disclosed for example in U.S. Pat. No. 4,522,464 issued to Thompson et al. The cable disclosed in the '464 patent provides an optical fiber enclosed in a steel tube disposed in the center of a well logging cable. A drawback to the cable disclosed in the '464 patent is that conductive members, positioned externally to the central tube containing the optical fiber, are constructed of copper clad steel wire in order to provide strength and inelastic strain resistance to the cable. Copper clad steel wire typically has different electrical impedance than does copper wire of similar electrical conductance. The conductor members in the cable of the '464 patent can be difficult to use for the electrical signal transmission schemes known in the art.
In another embodiment of logging cable disclosed in the '464 patent, one or more of the copper clad steel conductors can be substituted by optical fibers. A drawback to directly substituting optical fibers for conductor elements as disclosed in the '464 patent is that some of the electrical power and signal transmission capability of the logging cable will be lost since the substituted conductors are substituted by a non-conductive element, namely the optical fiber.
A further drawback to the cable disclosed in the Thompson et al '464 patent is that steel tube used to enclose the optical fiber is subject to inelastic strain and eventual failure as a result of repeated applications and relaxations of axial tension to the cable. The tube, positioned in the center of the cable as disclosed in the '464 patent, is subject to greater axial elongation under tension than any of the armor wires since the armor wires are helically wound around the axis of the cable and therefore enable elongation of the cable by unwinding of the helical lay of the armor wires under axial tension.
Another type of combination fiber/optic electrical well logging cable is described in "Manufacturing and testing of armored fiber optic downhole logging cable" by Randall et al, Wire Journal, September 1980. The cable disclosed in the Randall et al article provides plastic-sheathed optical fiber to replace one or more of the electrical conductors. A drawback to the cable in the Randall et al article is that the optical fiber is subject to fluid pressure in the wellbore since it is not pressure sealed. Another drawback to the cable in the Randall et al article is that some of the electrical conductors are replaced by optical fibers. The electrical transmission characteristics of a cable built according to the Randall et al design may not have suitable electrical transmission properties for use with certain well logging instruments.
Another combination fiber optic/electrical well logging cable is disclosed in international patent application number WO 94/28450 published under the Patent Cooperation Treaty. The cable disclosed in the WO 94/28450 application includes an optical fiber enclosed in a metal tube. The metal tube can be surrounded by braided copper strands which are used to conduct electrical power and electrical signals. An embodiment of the cable disclosed in the WO 94/28450 application includes application of the copper braids directly to the metal tube. A drawback to the cable disclosed in the WO 94/28450 application is that the tube is positioned at the center of the cable. Positioning the tube at the center of the cable, as previously explained, can subject the tube and the optical fiber to excessive axial strain under certain conditions. Furthermore, the cable disclosed in the WO 94/28450 application does not disclose or suggest a configuration of the metal tube and copper braids to provide electrical impedance characteristics similar to the insulated copper wires of the electrical well logging cables known in the art. In fact, the preferred embodiment of the cable in the WO 94/28450 application provides a layer of insulating material between the metal tube and the copper braids.
As is understood by those skilled in the art, well logging cables typically include electrical conductors and external armor wires which are respectively positioned to maintain a substantially round cross-sectional shape of the cable even after repeated applications and relaxations of substantial axial tension to the cable while further subjecting the cable to significant bending stresses. As is understood by those skilled in the art, the applications and relaxations of axial tension and bending stresses occur as a result of lowering the instruments into the wellbore and later removing them from the wellbore by winding and unwinding the cable through various sheaves which direct the cable into the wellbore from winch equipment provided for spooling and unspooling the cable. The well logging cables known in the art having only electrical conductors provide good maintenance of the round cross-section of the cable because all of the conductors have similar tensile and bending properties. Direct substitution of conductors with optical fibers to provide a logging cable having optical fibers will result in the cable having asymmetrical tensile and bending properties, and possibly reduced resistance to deformation of the circular cross-section of the cable.
Accordingly, it is an object of the present invention to provide a well logging cable having at least one optical fiber, the cable having mechanical configuration and mechanical properties similar to those of the electrical well logging cables known in the art.
It is a further object of the present invention to provide a well logging cable having at least one optical fiber in an assembly which has electrical power and signal transmission capabilities similar to the well logging cables known in the art.